A quantum dot (QD) is a nanocrystal or nanoparticle that ranges from 1-50 nanometers (approximately 5-250 atoms) in diameter or longest dimension. Quantum dots are composed of semiconductor material, however their small size results in characteristics that differ significantly from other semiconductor material. For example, the quantum containment effect of QDs produces emission and absorption spectra that vary with their composition, shape, and geometry. The quantum containment effect also produces discrete electron energy levels with transitions between states similar to atoms. As a result, quantum dots have unique and desirable behaviours that permit new technological and scientific applications. For example, the size of a quantum dot has a large impact on the energy bands of the dot, such that smaller quantum dots have higher band gaps and as a result produce higher energy light emissions during photoluminescence. Quantum dots are useful in optical, chemical, electronic, and photoelectrochemical applications, such as in solar cells, light emitting diodes (LEDs), and flat panel displays, and are of considerable value in the growing field of nanotechnology.
However, QDs are difficult and expensive to synthesize, particularly at an industrial scale. Known methods are either laboratory scale, suitable for obtaining small quantities of QDs (e.g., for research purposes), and/or they are expensive.
For example, QDs may be synthesized by using lateral patterns in remotely doped quantum wells or semiconductor heterostructures, thereby forming them from 2-dimensional electron or hole gases. These are essentially only of experimental interest with applications involving electrical currents.
In another process, spontaneous nucleation of self-assembled QDs occurs under specific conditions in both metallorganic vapour phase and molecular beam epitaxy. Islands of material thus formed become covered to make QDs. This method of manufacturing is expensive and is mainly of interest for quantum cryptography and quantum computation.
Viral assembly of QDs has been reported. Biocomposite structures were made using M13 bacteriophage viruses because such genetically engineered viruses can associate with semiconductor surfaces. Viruses can take the form of liquid crystals that are susceptible to change through their own concentrations in and the ionic strength of their solvent as well as applied magnetic fields. Therefore the association properties of viruses with other materials can be exploited through their liquid crystals to form inorganic nanocrystals such as QDs. This process is suitable for only very limited scale production.
In another example, electrochemical assembly of nanostructures such as QDs in a highly ordered fashion may occur spontaneously through ionic reactions at metal-electrolyte interfaces. These well ordered nanostructures can then be placed on a substrate from the metal surface. This process is used primarily as a way to study effects of the arrangement of the nanostructures.
In another example, high temperature dual injection is used to produce small quantities of QDs. This process is not practical for large scale production.
Larger quantities of QDs are produced in a highly scalable manner involving seed templates that are molecular clusters. Chemical precursors are converted into nanoparticles on these seed templates or points of nucleation which are stable. This method does not need the high temperature of the previously described dual injection system. However, the cost of this process is substantial. Another method of QD synthesis uses simultaneous increasing of the precursor concentration, but it requires further development.
What is needed is a more efficient and economical process for synthesizing quantum dots and nanoparticles.